1. Field of the Invention
The present invention relates to a hydraulic pressure control apparatus for controlling gear shifts, a transmission lock-up, or the like in an automatic transmission.
2. Description of the Prior Art
Automatic transmissions automatically effect gear shifts depending on the speed of automobiles on which the automatic transmissions are mounted, the loads on the engines of the automobiles, etc. Many automatic transmissions incorporate electric controls for controlling such gear shifts. It is well known in the art that hydraulic clutches and brakes (hydraulically operated engaging means) for controlling gear shifts are controlled by working oil that is supplied from solenoid-operated valves or the like.
Hydraulic pressure may be supplied from a single solenoid-operated valve to a single hydraulically operated engaging means. In such a configuration, the hydraulic pressure may be supplied from the solenoid-operated valve directly to the hydraulically operated engaging means, so that operation of the hydraulically operated engaging means can be controlled by controlling operation of the solenoid-operated valve.
Alternatively, a single solenoid-operated valve may be used to control operation of a plurality of hydraulically operated engaging means. More specifically, as shown in U.S. Pat. No. 4,875,391, for example, a single solenoid-operated valve serves to control operation of a low & reverse brake and a lock-up clutch of a torque converter. The low & reverse brake is a hydraulically operated engaging means (brake) which is engaged when the gear position is a 1st (low) or reverse position. The lock-up clutch is not engaged when the gear position is a 1st or reverse position. The lock-up clutch is engaged under certain conditions when the gear position is a 2nd or higher position (medium- or high-speed position). However, the low & reverse brake is not engaged in such a 2nd or higher position.
U.S. Pat. No. 4,875,391 discloses a directional control valve disposed in a hydraulic passage supplied with the hydraulic pressure from the solenoid-operated valve. The directional control valve is shiftable by a signal pressure for selecting a gear position. Depending on a gear position to be achieved, a signal pressure is applied to shift the directional control valve to supply hydraulic pressure from the solenoid-operated valve selectively to the low & reverse brake or the lock-up clutch. Specifically, when the 1st or reverse gear position is selected, the directional control valve is shifted to direct the hydraulic pressure from the solenoid-operated valve to the low & reverse brake to control operation of the low & reverse brake. When a medium- or high-speed gear position is selected, the directional control valve is shifted to direct the hydraulic pressure from the solenoid-operated valve to the lock-up clutch to control operation of the lock-up clutch.
Therefore, the single solenoid-operated valve can control operation of the hydraulically operated engaging means (low & reverse brake) for controlling gear shifts and also operation of the hydraulically operated engaging means (lock-up clutch) for controlling a transmission lock-up.
In the event of a failure of the directional control valve, however, any desired gear position cannot be achieved, making the control of the transmission unstable.
For example, when the spool of the directional control valve sticks and is held in a position to supply the hydraulic pressure from the solenoid-operated valve to the low & reverse brake, an attempt to actuate the lock-up clutch in a medium- or high-speed gear position causes the hydraulic pressure from the solenoid-operated valve to act on the low & reverse brake. Therefore, even though the lock-up clutch is to be actuated, the low & reverse brake is actuated instead. Since the low & reverse brake is actuated when a medium- or high-speed gear position is established, the transmission control becomes unstable.
Some hydraulic pressure control apparatus for automatic transmissions have a plurality of solenoid-operated valves corresponding respectively to clutches and brakes for controlling gear shifts. The clutches and brakes are supplied with working oil from the corresponding solenoid-operated valves to control their operation.
If one of the solenoid-operated valves fails to operate, then the clutch or brake which is supplied from working oil from that solenoid-operated valve cannot be controlled in operation. Therefore, in the event of a failure of one of the solenoid-operated valves while no hydraulic pressure is being supplied to a corresponding clutch or brake, the clutch or brake cannot be engaged, failing to achieve a corresponding gear position.
Automatic transmissions for automobiles usually have a plurality of gear positions in a forward range (e.g., a D range) and a single gear position in a reverse range. Consequently, even if a certain gear position cannot be established due to a failure of one of the solenoid-operated valves, the resultant problem may not be very serious because other gear positions can be established to make the automobile run. If the solenoid-operated valve corresponding to the reverse gear position fails to operate, however, the reverse gear position cannot be achieved, and the automobile cannot run backwards.
As well known in the art, the lock-up clutch is controlled by a control apparatus comprising a lock-up control valve for controlling the supply and discharge of a controlling hydraulic pressure for the control of operation of the lock-up clutch, and a solenoid-operated valve for controlling the supply and discharge of a pilot hydraulic pressure for the control of operation of the lock-up control valve (see, for example, Japanese laid-open patent publication No. 62-127552).
With the conventional control apparatus, when the solenoid-operated valve fails to operate while it is being actuated, the lock-up clutch remains engaged, holding the engine output shaft mechanically connected to drive road wheels. Therefore, upon stoppage of the automobile, the engine stalls.
One conventional solution has been to supply the pilot hydraulic pressure from the solenoid-operated valve to the lock-up control valve only in a medium- or high-speed gear position in which the lock-up clutch is to be engaged, and to prevent the pilot hydraulic pressure from acting on the lock-up control valve in a gear position such as a low-speed gear position or a reverse gear position in which the lock-up clutch is to be disengaged. To effect such hydraulic pressure control, the pilot hydraulic pressure may be supplied to the lock-up control valve through a shift valve that establishes a gear position in which the lock-up clutch is not to be engaged.
Automatic transmissions for automobiles establish a low-speed gear position when the speed of the automobile is low. When the automobile stops, it stops while the automatic transmission is in a low-speed gear position, e.g., a 1st gear position. According to the above conventional solution, therefore, when the automobile stops in the event of a failure of the solenoid-operated valve while it is being actuated, since the selected gear position is a low-speed gear position upon automobile stoppage and no pilot hydraulic pressure acts on the lock-up control valve, the lock-up clutch is disengaged regardless of whether the solenoid-operated valve is actuated or not. Consequently, the engine is prevented from stalling.
As described above, the engine is prevented from stalling upon a failure of the solenoid-operated valve. However, if the lock-up control valve itself fails to operate and remains shifted to engage the lock-up clutch, e.g., if the spool of the lock-up control valve sticks against movement from the position in which hydraulic pressure is supplied to engage the lock-up clutch, then the engine is not prevented from stalling.